Sarah Polley frowns. We're seated opposite each other in the conference room of a Beverly Hills boutique hotel, and I've just asked what I thought was an innocuous question about her new documentary, Stories We Tell. "I'm hesitant to talk to you about that specific thing," she replies to the query in question. "It's such a spoiler." That's the sort of answer you might expect to get from an actor guarding superhero movie secrets, but not from Polley, a 34-year-old actress-cum-director who's made an indie documentary about the skeletons in her family closet.

But Stories We Tell (which opens today in New York, L.A., and other select cities) is no ordinary documentary, and that's by design. (I'll tread lightly when describing the plot, so as not to spoil anything that Polley didn't put in the trailer herself.) The movie begins as an investigation of Polley's mother, Diane, who died when Sarah was 11. As we learn more about Mom — a vivacious social butterfly who put her acting career on ice to raise a family — Polley begins to reveal her real motivation for making the movie: She's long suspected that she was the result of an affair Diane kept secret, which means that her charming father Michael may not actually be her real dad.

The secrets uncovered by Polley give Stories We Tell its spine, but she's even more interested in how those secrets are told, and how what seems like an absolute truth to one family member can be contradicted by another. A working actress since the age of four — she's best known for The Sweet Hereafter and Zack Snyder's 2004 remake of Dawn of the Dead — Polley has been around long enough to know that she could wield the movie camera like a trump card, telling her family's history as though her take is the definitive version, and what's fascinating about Stories We Tell is how she undercuts herself every time her side of the story begins to feel authoritative. She freely admits that one family member stopped speaking to her when she pressed ahead with the movie — he felt that only he could tell this particular story correctly — and there are a few surprise reveals late into Stories We Tell that make you question whether Polley is telling us the whole truth at all.

"Because the film was so much about storytelling and how stories are constructed, it would have felt really false to me to leave out the fact that I was constructing this story — and that this, in itself, was very subjective," explains Polley. Yes, this means that she puts herself in front of the camera, but don't mistake her for spotlight-hogging documentarians like Michael Moore or Morgan Spurlock. In one key Stories We Tell sequence, Sarah sits in a recording studio, coolly supervising her father Michael as he tells his side of the story. After he finishes one particularly fraught passage, Sarah demands a retake: She wasn't a fan of his line readings. She's not the first documentary filmmaker to ask her subject to redo an emotional moment, but most directors would leave that sausage-making scene out, lest it damage their brands.

"I felt really uncomfortable including that," she admits. "Especially because I wanted to be as much 'myself' as I could, which meant not looking the way I would like to look in a movie, and not behaving necessarily the way I would want people to see me behave."

So what inspired Polley to be so transparent about her process? Well, it didn't hurt that she's spent three decades serving as the subject of profiles like this one. "Having been put in the position of being represented in ways that I didn’t feel were rigorous enough, or accurate, I think I was kind of conscious of my role in doing that [to other people]," she says carefully. "I think I was very attuned to how vulnerable people were by speaking to me on camera, and I definitely wanted to be as ethical as I possibly could in terms of how they were represented. I probably agonized over that a little bit more than another filmmaker might have."

One thing Polley no longer agonizes about is her acting career: Before making Stories We Tell, she wrote and directed the 2006 Alzheimer's drama Away From Her (which earned her an Oscar nomination for Best Adapted Screenplay) and last year's Take This Waltz, starring Michelle Williams and Seth Rogen. "Acting is definitely not a priority, but I will do it every now and then," Polley says. "I’m acting in a Wim Wenders film starting in the summer and I’m really looking forward to that, but it’s never going to be the thing that drives me." I ask Polley whether she still allows her representatives to put her name on the actor-availability lists that directors will consult when casting a new project. "No," she says. Then she laughs at how quickly she gave her blunt answer, adding, "I've firmly taken myself out of that life."

Still, she wouldn't mind doffing her filmmaker hat for just a moment if it would give her a better perspective on the knotty, personal movie she's just made. "One of the things I wish I could do in my life would be to watch this film through somebody else’s eyes," she says. "I just can't. I still see it as just a giant mess, and other people are seeing that it has a shape. That's really exciting, because I still have a hard time seeing it clearly."

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